The coating of steel components with aluminum-based coating alloys, commonly referred to a hot dip coating, is well known in the prior art. One particular type of coating is trademarked as Galvalume®, which is owned by BIEC International, Inc., and is representative of an aluminum-zinc coating alloy.
These materials are advantageous as building materials, particularly wall and roof construction due to their corrosion resistance, durability, heat reflection, and paintability. Typically, these materials are manufactured by passing a steel product such as a sheet or plate through a bath of a melted alloy coating composition comprising aluminum, zinc, and silicon. The amount of coating applied to the steel products is controlled by wiping, and then the products are cooled. One characteristic of the coating applied to the steel product is its grain size or spangle facet size.
U.S. Pat. No. 3,343,930 to Borzillo et al., U.S. Pat. No. 5,049,202 to Willis et al. and U.S. Pat. No. 5,789,089 to Maki et al. disclose methods and techniques for the manufacture of steel sheets coated with these aluminum-zinc alloys. The three references are herein incorporated by reference in their entirety.
European Patent Application No. 0 905270 A2 to Komatsu et al. discloses another coating process utilizing zinc, aluminum, and magnesium. This application is directed at solving the corrosion problems associated with baths containing magnesium as an alloying element. Further, it is disclosed that the undesirable stripe pattern occurring in magnesium-containing baths does not occur in baths without magnesium.
U.S. Pat. No. 5,571,566 to Cho discloses another method of manufacturing coated steel sheet using an aluminum-zinc-silicon alloy. The object of the Cho patent is to provide a more efficient production method for manufacturing coated steel sheet. Cho meets this object by uniformly minimizing the size of spangles by introducing a large number of spangle particles into the coating, which limits subsequent growth of the spangles because these particles interfere with their respective growth resulting in a smaller spangle facet size. The seed effect is achieved by using titanium as part of the molten coating composition.
A similar disclosure with respect to the use of titanium in coating baths to minimize spangle facet size is disclosed in an article entitled “Minimization of Galvalume Spangle facet size By Titanium Addition To Coating Bath”, by Cho, presented for the INTERZAC 94 Conference in Canada in 1994. In this article, the author indicates that elements such as titanium, boron, and chromium produce finer spangles in a Galvalume coating, such a disclosure consisted with the disclosure of the Cho patent.
Another disclosure, Japanese Patent Laid-Open Publication No. S62 (1987)-023976 to Yukio, et al., is directed to Zn—Al bath additions that include the alloying elements (Ti, B, Nb, etc.) in elemental form. Such elemental form additions cause a reaction with the Al in the Zn—Al melt to create Al—Ti, Al—B, etc. particles that act as nucleation sites for the spangle. For Ti, the process is exactly the same as that claimed by Cho, although Cho does not indicate particle formation in the melt.
In the present invention, as described in the “Detailed Description of the Preferred Embodiments,” we add grain the refining particles Ti—B, Al—B, Ti—C, etc. directly to the melt to achieve grain refining. Our grain refining particles provide improved results over Yukio in that his spangle size is at least 2.5 times greater than our spangle size, and therefore, the Yukio spangle is visible to the naked eye. It is not a spangle-free product as taught by the present invention. Furthermore, Yukio indicates that making alloy additions outside his cited range leads to particle coarsening and loss of effectiveness. We are able to add large amounts of the above mentioned grain refining particles with no increase in spangle size. Accordingly, the present invention is an improvement over the Yukio teaching because pot factors that impact the particle forming reaction in the Japanese disclosure do not affect the present improved grain refining method and coated product. A person having ordinary skill in the art would not expect that grain refining particles Ti—B, Al—B, Ti—C, etc. added directly to the Zn—Al bath, as taught in the present invention, would be more effective than creating Al—X particles in situ as taught by Yukio. In addition, one skilled in the art would not expect the different Ti—B, Al—B, Ti—C, etc. particle chemistry to be more effective than the Al—Ti, etc. (all Yukio particles contain Al) particle chemistry taught by Yukio.
In yet another disclosure published in the Fourth Australian Conference on Nuclear Techniques of Analysis, an article by Mercer, et al. entitled “Some Applications of Electron Spectroscopy in the Sheet Metal Industry,” provides a brief discussion of Al—Zn coating grain size in section 2.3 (page 135). However, based on an Interview Summary in the above listed priority application Ser. No. 10/753,099, the Examiner stated that Mercer does not constitute prior art that can be cited against the patent claims in the above listed priority U.S. Pat. No. 6,468,674. The Examiner believes that the Mercer, et al. article does not establish a working knowledge about the intentional usage of boride constituents in an Al—Zn coating bath, and the article does not show or reasonably suggest any benefits of using boride constituents. According Mercer should not be considered prior art in the present invention.
Notwithstanding the improvements suggested by Cho and Yukio, presently used coated steel products still have disadvantages. One disadvantage is that, when the coated steel product is to be painted, a temper rolling is required to flatten the product in preparation for painting. Another problem is cracking when the product is a sheet and is bent. When this sheet product is bent, the coating can crack, the crack exposing the steel to the environment and premature corrosion. With presently available coated steel sheets, large cracks can form, thereby compromising the corrosion resistance of the sheet product.
In light of the deficiencies in the prior art, a need has developed to provide an aluminum-zinc coated steel product with improved bending performance, reduced spangle facet size, and improved painted surface appearance. The present invention solves this need by providing a method of coating a steel product, a coating composition and a coated steel article which, when experiencing surface cracking during bending, is still corrosion resistant and does not require temper rolling when the coated steel product is painted. The coating composition is modified with one or more particulate compound constituents such as titanium boride, aluminum boride and the like.